How to Predict Excited State Geometry by Using Empirical Parameters Obtained from Franck‐Condon Analysis of Optical Spectrum

• Published in: Chemphyschem Vol. 22; no. 20; pp. 2078 - 2092
• Main Author: Köse, Muhammet Erkan
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 14.10.2021
• Subjects: Absorption spectra; Empirical analysis; Excitation; Excitation spectra; excited states; Franck-Condon approximation; Naphthalene; optical spectrum; perylene; Photoexcitation; pyrene; simulation; Wave functions
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.202100437

Excited state geometries of molecules can be calculated with highly reliable wavefunction schemes. Most of such schemes, however, are applicable to small molecules and can hardly be viewed as error‐free for excited state geometries. In this study, a theoretical approach is presented in which the excited state geometries of molecules can be predicted by using vibrationally resolved experimental absorption spectrum in combination with the theoretical modelling of vibrational pattern based on Franck‐Condon approximation. Huang‐Rhys factors have been empirically determined and used as input for revealing the structural changes occurring between the ground and the excited state geometries upon photoexcitation. Naphthalene molecule has been chosen as a test case to show the robustness of the proposed theoretical approach. Predicted 1B2u excited state geometry of the naphthalene has similar but slightly different bond length alternation pattern when compared with the geometries calculated with CIS, B3LYP, and CC2 methods. Excited state geometries of perylene and pyrene molecules are also determined with the presented theoretical approach. This powerful method can be applied to other molecules and specifically to relatively large molecules rather easily as long as vibrationally resolved experimental spectra are available to use. Table of Contents Text: Excited state geometries of molecules are predicted by using experimental absorption spectrum and Franck‐Condon analysis of the vibrational peaks. Predicted excited state geometries are similar to those obtained with quantum mechanical calculations. Fast and facile estimation of excited state geometries for even relatively large molecules opens up an avenue to generate a data set that can be used in benchmarking studies for quantum mechanical methods focused on excited states.